Electrostatic precipitator



March 28, 1961 J. 5. LAGARIAS 2,976,951

ELECTROSTATIC PRECIPITATOR Filed Oct. 17, 1958 2 Sheets-Sheet 1INVENTOR. 51 JOH /S. Leseleras BEMQW March 28, 1961 J. 5. LAGARIASELECTROSTATIC PRECIPITATOR 2 Sheets-Sheet 2 Filed 001. 17, 1958 CURRENTVOLTAGE. CHARACTER ISTICS OF H VW PZECIFITATOZ VOLTAGE I N K V INVENTOR. JOHA/ S. Lasezres.

69W, TTOEI/QY United States PatentO 2,976,951 ELECTROSTATIC PRECIPITATORJohn S. Lagarias, Pittsburgh, Pa, assignor to Koppers 1 I This inventionrelates generally to electrostatic precipitators and more particularlyto a method and ap paratus for removing material deposited on theelectrodes of electrostatic precipitators.

It has become conventional practice to remove the particles which areentrained in a gas stream from the gas stream by the use ofelectrostatic precipitators. Such precipitators have suspended in thegas stream two sets of electrodes, a corona electrode and a collectorelectrode. As the gas passes these electrodes, it is ionized and theentrained particles are given an electrical charge thereby so as to beattracted mainly to the collector electrode. The clean gas leaves theprecipitator and the collected material is removed to a hopper andthereafter re moved from the precipitator.

The removal of the deposited material from the electrodes areconventionally of two general types: the impulse type, wherein a hammeris actuated periodically by a spring, magnetic or pneumatic actuatingmeans against an anvil to cause an impulse and the vibrator type,wherein a plunger is reciprocated rapidly against an anvil connected tothe electrodes by means of spring, magnetic or pneumatic actuatingmeans. I

It is desirable that the dust be dislodged from the electrodes in theform of large agglomerates of such size that they fall against the duststream into the dust'hopper.

Therefore, the rapping blows should not be so powerful as to dispersethe dust and increase the re -entrainment or,;stated differently, sincethe material from the electrodes'ca'n be re-entrained, the magnitude ofthe rapping blowsshoud be as light as possible because re-entrainment ofthe dust will considerably efiect the efl'iciency of the precipitator.Additionally, heavy blows of high intensity may damage the electrodesand their supporting structure.

The mechanism which carries the particles of entrained material from thegas stream to cling to the precipitating electrodes and which causes theparticles of material to cling to each other so as to form agglomeratesis not .of the current flow through the dust results in a pasting effectwhich holds the dust more rigidly to the plate than it'would adhere ifthere isno'current fiowpresent. The

glegree of adherencedepends uponsuch factors as the "ice intensity ofcurrent flow, the electrical conductivity of the dust and the degree andtype of packing which the dust undergoes. To cause the material to bedeposited at all on the electrodes, the potential supplied between theelectrodes must generate a corona-effect which is great enough to ionizethe gas between the electrodes and cause a current flow through the gasbetween the electrodes. Under normal conditions in a typicalprecipitator, for example, the potential difierence which causes theseeffects may be about 60 kilovolts; and in such a case, if the potentialdiiference be reduced to 25 kilovolts or less no current will flowbetween the electrodes and the ions already present in the gas streamand on the pre cipitated material will flow to the grounded electrodes.

According to this invention, the rapping of the electrodes is eifectedby vibrations of low intensity when the potential between the electrodesis momentarily reduced to a predetermined level thus effecting thecleaning of the electrodes without damage to the electrodes and withoutappreciable re-entrainment in the gas stream. Accordingly, thisinvention provides a novel process and apparatus for reducing thepotential diiference between the electrodes to the extent necessaryduring the period the electrodes are being rapped so that no furthercurrent flows between the electrodes and thereafter raising thepotential difference to the predetermined optimum so that current flowbetween the electrodes again takes place,

thereby obtaining the advantage of making the change in potential asslight as possible so that the power loss is minimized.

The above and further novel features of the invention will appear morefully from the detailed description when the same is read in connectionwith the accompanying :drawings; It is to be expressly understood,however,

that the drawings are not intended as a definition of the invention butare for the purpose of illustration only.

another embodiment of the rappers of Fig. 1.

Fig' 5 is a current voltage curve of a typical electrostaticprecipitator.

An electrostatic precipitator 11 is shown (Fig. 1) with its shellremoved. For the sake of simplicity only two preoipitator lanes areshown, although advantageously a plurality of precipitator sections areprovided in parallel. Conventionally, each section is provided with twosets of electrodes 13 and 15; Set 13 is advantageously in the form ofwires which are suspendedfrom' a supporting member '17 that is supportedin the precipitator shell from suitable insulating means (not shown),while set 15 is advantageously in the form of plates suspended from asupporting member 21 supported from the precipitator she'll. At thebottom of electrodes 13- are weights 19 which properly distend theelectrodes 13 in the precipitator shell whereas no weights are necessaryto distend the plates 15. i

Electrodes 13 are given a high potential with respect to electrodes 15which may be grounded so that a corona current is produced therefrom andthus, as dirty gas passes into the vprecipitatorbetween the electrodesdescribed, the gas is ionized and the dust entrained in the gas thuspicks up a charge so that'it is mainly attracted to collectingelectrodes 15. .Clean gas then passes out the precipitator. Thepotential to electrodes 13 and 15 is supplied from aconventionalalternating power source 28 through a circuit comprised of aconventional voltage regulator ;3l-,-.lead=29, contact 32, armature 33,lead 35,

aura-es 1 primary winding 37 of step-up transformer .39, ,lead 41,junction 43 and lead 45. Stepped-up voltage from secondary winding 47 ortransformer 39 is supplied through leads 4 3- and '52,conventionalrectifier tubes 51 and 53, and leads 57 and 63 as a pulsating directcurrent so as to produce a negative potential to -electrodes 13 and apositive potential to electrodes 15 relatively between the electrodes.Junction 61 of lead 63 may be connected to ground by means of suitableconnections (not shown) connected to the precipitator shell which isgrounded.

It has been found, as described above, that to cause material to bedeposited on the electrodes, the potential supplied to the electrodesmust be high enough to generate a corona current. Under normalconditions in con ventional precipitators the potential difference isabout 60 kilovolts to generate the required corona. Such a corona isadvantageously just below the level at which excessive sparking occursbetween electrodes 13 and 15.

.In such sense, if the voltage is reduced, the current flow fromelectrodes 13 to electrode 15 will be reduced as illustrated in Fig. 5.Also, under normal conditions, a certain amount of voltage Ec must beimpressed on electrodes 13 in order to ionize the surrounding gas andproduce a flow of current from electrodes 13 through the ionized gas toelectrodes 15. Thus when that voltage is reduced below a certain levelthere is no flow 'of current switching systems, to be described in-moredetail hereinafter, may be applied to the primary circuit during thevoltage interruption period to cause the voltage to fall.

If the time'constant of the precipitator is not excessive the normalcharacteristics of the power supply will reduce the voltage below Berapidly enoughso that no additional load is necessary.

Fig. 3 shows a circuit with a .timing'controlifor sequentially loweringand raising the voltage at predetermined intervals. To this end, aconventional timer 65 is'provided with a suitable power source 66 forenergizing relay coil 71 in a predetermined sequence through leads 67and 69. Thus when leads'67 and 69 are energized, relay'coil 71 pullsannatures 33 and 73 downwardly.

When armature 33 is pulled downwardlyit disengages contact 31 andengages contact 75. Thus high voltage current from'source 28 no longerflows from 'lead 29 to lead 35 and low voltage current begins toflowfrom lead 77 through contact 75, armature 33, lead 35, coil 37 andlead 41 back to a 220 volt source of current 28a to efiectively lowerthe voltage on transformer 39. This relay coil 71 may be part of aconventional contactor, typical of which is the Westinghouse-type15-825-NT45A. When relay coil 71 is de-energized this pulls armature .33upwardly to re-engage it with contact 31; Thus, by

varying the primary voltage input, the voltage supplied to transformer39 is raised and lowered in a sequence so that corona current isgenerated by the electrodes 13 and 15, removed and then reinstated.

The duration of'the voltage reduction period depends upon the thicknessof the dust, the resistivity of the dust, and the movability of thecharged particles entrained in the dirty gas stream. The dust movementdepends on the location of the charged particle, particle size and fieldstrength. Most of the current is not carried by the dust but by thegaseous ions which move through the gas in approximately 0.01 ofasecond. Through the dust,1 however, the flow of current is iess rapid.

At steady state the current is flowing through the dust at a. uniform.rate. If the voltage is interruptedthe rate 4 of .currentflow would beapproximately that of the time constant RC involving the capacitance andresistance of the dust. The time constant is a measure of the rate ofvoltage fall and may be approximated to give the rate of current decay.

Based on test results of such current decay, it is estimated that theinterruptionpjeriod may vary between 0.02 to 5.0 seconds depending uponthe material of the dust and the rapping efiect desired. 'It'isconsistent that for very conductive dust the interruption of the voltagemay be sufiicient whereby no .zrapping .atall will be required to removedeposited material from the electrodes.

Normally, in accordancewith this invention, rappers 25 (Figs. 1 and 3)vibrate electrodes 13 and 15 to remove material deposited thereon. Thiscauses the deposited material to be removed from the electrodes and thedeposited material then falls by gravity into a hopper (not shown) atthe bottom of the precipitator. For'this purpose, advantageouslypneumatic reciprocating rappers 25, as shown in Fig. 2, which areattached to the electrodes, as shown in Fig. l, are actuated. To thisend an air sup ply (notshown) is attached to the rapper housing 78 so asto reciprocate rapidly hammer 79 against anvil 80 connected to thesupporting members 17 and 21 .respectively by suitable connecting rods81. The intensity of the rapper vibrations are easily adjustable. Forexample, the air volume and pressure supplied thereto from a suitableair source (not shown) may be increased or decreased.

When pneumatic reciprocating rappers (Fig. 3 are used in accordance withthis invention, advantageously air is supplied to the rappersintermittently. To'this end, the conventionaladjustable electrical timer65 is set to energize solenoid 71 at predetermined intervals, asdescribed above. Solenoid 71 when energized closes armatures :33 and 73with contacts 75 and 82 respectively. The engagement of armature 73 withcontact 82 energizes solenoid 83 which actuates two-Way valve 91 tocause air to be supplied to rappers 25 causing vibrations to be impartedtotheelectrodes 13 and 15. When armature 73 is disengaged from contact82 this actuates the twoway valve 91 to cut oif the air supply to therappers causing-the vibrations to these electrodes to cease.

in the normal operation of the electrostatic precipitator 11 accordingto ;this'invention,-the voltage supplied to the discharge electrodesfrom the 440 volt current source 28 is-typically about 60 kilovolts. Atthis'voltage there is a current flow of about 6.75 milliamperes throughthe gaseous ions moving through the gas and through the dust layerdeposited on the precipitator electrodes. This current flow is primarilythrough the gas but the small current fiow through the dust apparentlycauses a cementing action which tends to hold deposited dust on theelectrodes with great'tenacity.

The voltage is then decreased sequentially for appropriate timeintervals depending,.for example, on the dust layer on the electrodes,therresistivity of the dust and the mobility of the charged particlesbelow that voltage which causes corona around the precipitatorelectrodes. To this end at appropriate-intervals timer 65 causesarmature 33 to disengage contact 31 and to engage contact 75 so that 220volt current is supplied to the precipitator electrodes from a suitablesource (not shown). The time 'is:no:current :flowthrorighthe dust layerthe cementing action is snbstantially'removed and the vibration in theelectrodes 1 increases the cleaning action of the electrodes. 'l hus thelow intensity rapping of-thisinvention removes reassess inaterinrrsm'the pree' itator'electrodes f-when tlie vatage' on '--'theelectrodes is "reduced in accorda'nce with this invention. Thus thecleaning action of the rappingis increased and the overall efifect is toeffectively clean the electrodeswhilereducing the r'e-entr'ainr'n'entofmaterial cleaned from the electrodes. g

It is understood by one skilled in the art, from the above, that usingone 440 volts power source and a resistance or reactance connected inseries between leads 29 and 77 may operate aswell "as'when using 220and440 volts power sourcesbut it is-inoie'economical to switch from onevoltagetoanother asdescribed.

A further embodiment is also contern'plated by this invention using thedouble contact switch described above. This embodiment incorporatesanimpulse rapper-such as a magnetic rapper 25a shown in'Fig. 4'which' isenergized :atthe same time that the voltage on the precipitator elec-"trodes is reduced. Tothis end-armature l3 iscorinectedso that when itengages contact 82 current is suppliedfrom ia conventional power sourcethrough the rapper coil to actuate the hammer therein. Thus in thisembodimentas "well as in the last mentioned above, the opening andclosing of the armature 73 takes place during the period of voltageinterruption so that the cleaning of the electrodes takes place duringthe voltage'interruption.

It is understood, however, that a separate timer from timer 65 may beused for'theclosing and opening ofarmature 73 in which case rapper 25maybe caused to perate in like manner as described above before,during,a'nd after the voltage reduction which 'was caused in a manner asdescribed above. v I The foregoing has described novel apparatus andmethod for electrode rapping. In accordance with this invention, voltageis reduced and the electrodes vibrated whereby the entrained materialwhich has collected on the electrodes is effectively removed as aglomerates under blows which are relatively light and 'whichdo not causeappreciable damage to the electrodes or re-entrainment of the materialin the gas.

What is claimed is:

1. Apparatus for cleaning electrodes in an electrostatic precipitatorhaving first and second 'electrodes' 'suspended therein onwhichparticles are deposited from a dirty gas comprising a source ofhigh current flow for supplying a predetermined first direct electricalvoltage of onelpolarity to said first electrodes to produce a potentialdifierence between said first and second electrodes which is just belowthe level at which excessive sparking occurs between ;-said first andseconde'le'ctrodes and which is great-enough 1to ionize' said gas'and'to' transmit electrical current be- :tween said first and secondelectrodes, said particles being :charged when said current istransmitted so that said par- .ticles migrate to said electrodes and aredeposited thereon,

.a source of low current flow for supplying a predetermined seconddirect electrical voltage of the same polarity to said first electrodesto produce a potential difference between said first and secondelectrodes which is just below the level which produces current to betransmitted between said first and second electrodes by means of saidionized gas, means for automatically and alternately connecting saidhigh and low source of current flow so as to cause said first and seconddirect electrical potential differences to be selectively suppliedbetween said first and second electrodes, and vibrating means connectedto said first and second electrodes operative when said second directelectrical potential difference is supplied between said first andsecond electrodes for vibrating said electrodes so as to removeparticles deposited thereon.

2. The invention of claim 1 wherein said vibrating means is' a pneumaticactuating rapper which produces continuous low intensity vibrations insaid electrodes.

3. The invention of claim 1 wherein said vibrating means is a magneticimpulse rapper which produces inter- ,mittent low intensity vibrationsin said electrodes.

4. Apparatus for cleaning electrodes in an electrostaticprecipitatorlthaving adischarge-electrode :and 'a collecting electrodecomprising a 'first alternating 'electrical energy source of apredetermined firstvoltage; a second alternating electrical energysour'ce of :a predetermined second :voltage'whi'ch is less than?saidfirstipredeterinined voltage, common means' 'forstepping-up said firstand second predetermined voltages andsupplying them to said dischargeelectrode, and means for alternately connecting said common meanswithsaidfirst and second alternatingelec- 'trical energysources so as toalternately :supplvpredetermined third 'and'fourth voltages of the-samepolaritymespectively to said discharge electrode and means operative forvibrating said collecting electrodes only when-said fourthvoltage issupplied thereto.

'5. In an electrostatic precipitatorhaving first and' secon'delectrodes, said first electrodes being adapted tohaye voltage impressedthereon so' as to cause material entrainedjin'a dirty gas to beaccumulated primarily on said "second electrodes, apparatus for changingthe voltage 'supplied'to said first electrodes comprising a firstalternating electrical 'energysource of predetermined relatively highfirst voltage, a second alternating electrical energy source ofpredetermined relatively low second voltage, common means forstepping-up and rectifying "said first occurs between s'aid first andsecond electrodes andzbeing great'enough to causesaidentrained materialto accumulatcprimarilyonsaid second electrodes, said 'fourth' voltagebeing just below the level which causes said'entrained materialto-accumulate on said second electrodes, means having a double contactswitch for selectively connecting said common means alternatelywith'said first and second 'saidfourth voltage issupplied to said firstelectrodes'said material does not-accumulate on said electrodes and maybe cleaned easily therefrom and means operative for vibratingsaidelectrodes when said fourth voltage is supplied to s'aidfirst'electrode. 7

*6. In combination with an electrostatic precipitator havin'g electrodeswhich are adapted to accumulate material 'fr'ornagas having entrainedmaterial therein, a cleaning systemcornpris'inga first source of currentfor supplying a predetermined first electrical voltage of one polarity.:to saidelectrodes to p'roduce aforce which is great enough to causematerial entrained in said gas to accumulate on said electrodes, asecond source of current for supplying a predetermined second electricalvoltage of the same polarity to said electrodes to produce a force whichis just below the force necessary to cause material entrained in saidgas to accumulate on said electrodes, means for selectively causing saidfirst and second sources of current to alternately supply said first andsecond predetermined electrical forces to said electrodes, and vibratingmeans connected to said electrodes operative when said secondpredetermined el'ectrical force is being supplied to said electrodes forvibrating said electrodes so as to remove material accumulated on saidelectrodes.

7. In an electrostatic precipitator having high and low potentialelectrodes wherein a high potential difference is impressed between saidelectrodes so as to cause ionization of the surrounding gas and whereinsaid electrodes are adapted to be suspended in a gas stream havingparticles entrained therein so that there is a current flow between saidelectrodes due to ionization of said gas stream by said high potentialand wherein said particles are charged so that they are caused todeposit on said electrodes, a source of high current flow and a sourceof low current flow apparatus for selectively connecting said high andlow current flow sources thereby increasing and decreasing the potentialbetween'said electrodes whereby current flow between said electrodes isreduced and whereby low intensity vibrations in said electrodes willefiectively remove particles deposited thereon comprising means forvibrating said electrodes with low intensity vibrations, first meanshaving a first alternating electrical energy source of a first tensionfor supplying a first high voltage of one polartiy to said highpotential electrode which is just below the voltage which causesexcessive spark-over between said electrodes so as to cause high currentflow between said electrodes, second means having a second alternatingelectrical energy source of lower tension than the tension of said firstalternating source for supplying a second voltage of the same polarityto said high potential electrode which is just below the voltage whichcauses current to flow between said electrodes, and means forselectively connecting said first and second means with said highpotential electrode at predetermined time intervals whereby said currentflow is reduced and particles deposited on said electrodes are removedfrom said electrodes by said low intensity vibrations on saidelectrodes.

8. An electrostatic precipitator for cleaning foreign matter from a gasstream having foreign matter entrained therein, comprising first andsecond electrodes suspended in said gas stream, vibrating means forproducing low introdes, means for alternately connecting said first andsecond means to said first electrodes, and means for actuating saidvibrating means to remove matter accumulated on said electrodes whensaid second'means is connected to said first electrodes whereby matteraccumulated on said electrodes can be removed therefrom by low intensityvibrations.

9. An electrostatic precipitator for cleaning foreign matter from a gasstream having foreign matter entrained therein, comprising first andsecond electrodes suspended in said gas stream, vibrating means forproducing low intensity vibrations attached to said electrodes, firstmeans for supplying a first voltage of one polarity to said firstelectrodes so as to produce a first high current fiow between said firstand second electrodes which causes said foreign matter to be accumulatedon and to cling strongly to said electrodes, second means for supplyinga'second lower voltage of the same polarity to said first electrodes soas to reduce said first high current flow whereby said matteraccumulated on said electrodes clings less strongly thereto, means foralternately connecting said first and second means to said firstelectrodes, and

means for actuating said vibrating means to remove matter accumulated onsaid electrodes during and after the connection of said second means tosaid first electrodes whereby matter accumulated on said electrodes canbe removed therefrom by low intensity vibrations while still preventingsaid gas from passing through' said precipitator Without being exposedto a current flow between said first and second electrodes.

10. An electrostatic precipitator for cleaning foreign matter from a gasstream having foreign matter entrained therein, comprising first andsecond electrodes suspended in said gas stream, vibrating means forproducing low intensity vibrations attached to said electrodes, a firstsource of high current flow for supplying a first voltage of onepolarity to said first electrodes so as to produce a first high currentflow between said first and second electrodes which causes said foreignmatter to be accumulated on and to cling strongly to said electrodes, asecond source of low current flow for supplying a second lowervoltage ofthe same polarity to said first electrodes so as to produce a secondlower current flow between said first and second electrodes wherebymatter accumulated on said electrodes clings less strongly thereto, andmeans for alternately connecting said first and second sources ofcurrent flow to said first electrodes and having means for actuatingsaid vibrating source of current flow when said second means isconnected to said first electrodes whereby matter accumulated on saidelectrodes can be removed therefrom by low intensity vibrations whilepreventing said gas from passing said electrodes without being exposedto a current flow between said first and second electrodes.

References Cited in the file of this patent UNITED STATES PATENTS2,247,361

UNITED STATES PATENT OFFICE CERTIFICATIN 0F QGRECTION Patent No,2,976351 March 28, 1961 Johns, Lagarias It is hereby certified thaterror eppears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below Column 1, line54, for "shoud" read should -g line 59, for "carries" read causes Signedand sealed this 5th day of September 1961a (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents

